The story is organized into seven brief chapters. The first describes Microm gas (small/large), an inhabitant of one of the planets that orbits Sirius. His home world is 21.6 million times greater in circumference than the Earth. Microm gas stands 120,000 feet (about 23 miles) tall. When he is almost 450 years old, approaching the end of his infancy, Microm gas writes a scientific book examining the insects on his planet, which at 100 feet are too small to be detected by ordinary microscopes. This book is considered heresy, and after a 200-year trial, he is banished from the court for a term of 800 years. Microm gas takes this as an incentive to travel around the Universe in a quest to develop his intellect and his spirit. After extensive celestial travels he arrives on Saturn, where he befriends the secretary of the Academy of Saturn, a man less than a twentieth of his size (a "dwarf" standing only 6,000 feet tall). They discuss the differences between their planets. The Saturnian has 72 senses while the Sirian has 1,000. The Saturnian lives for 15,000 Earth years while the Sirian lives for 10.5 million years; Microm gas reports that he has visited worlds where people live much longer than this, but still consider their lifespans too short. At the end of their conversation, they decide to take a philosophical journey together. Eventually, they arrive on Earth and circumnavigate it in 36 hours, with the Saturnian only getting his lower legs wet in the deepest ocean and the Sirian barely wetting his ankles. They decide that the planet must be devoid of life, since it is too small for them to see with the naked eye. In the Baltic Sea, the Saturnian happens to spot a tiny speck swimming about, and he picks it up to discover that it is a whale. As they examine it, a boatful of philosophers returning from an Arctic voyage happens to run aground nearby. The space travellers examine the boat and, upon discovering the lifeforms inside it, they conclude that the tiny beings are too small to be of any intelligence or spirit. Yet they gradually realize the beings are speaking to each other, and they devise a hearing tube with the clippings of their fingernails in order to hear the tiny voices. After listening for a while, they learn the human language and begin a conversation, wherein they are shocked to discover the breadth of the human intellect. The final chapter sees the humans testing the philosophies of Aristotle, Descartes, Malebranche, Leibniz and Locke against the travellers' wisdom. When the travellers hear the theory of Aquinas that the universe was made uniquely for mankind, they fall into an enormous fit of laughter. Taking pity on the humans, the Sirian decides to write them a book that will explain the point of everything to them. When the volume is presented to the Academy of Science in Paris, the secretary opens the book only to find blank pages.